This invention relates to improvements in an engine knocking control unit, and more particularly, to an engine knocking control unit adapted to prevent erroneous control when an engine is cold.
The art disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 60-75730 is an example of a conventional engine knocking control unit. In accordance with this prior art, a knocking control unit includes knocking sensing means for sensing engine knocking, and correcting means which receives an output from the knocking sensing means for correcting an engine control variable, such as the ignition timing of the fuel mixture, in a direction that will suppress knocking when knocking occurs. (More specifically, this entails retarding ignition timing.) This conventional unit suppresses knocking effectively to assure engine durability.
In a case where the control variable is constituted by e.g. ignition timing for igniting a fuel mixture, correcting the ignition timing by retarding the same is accompanied by a change in the state of the combustion of the mixture. Ordinarily, therefore, it is required that a maximum allowable amount of correction be preset. If the amount of correction of ignition timing exceeds the preset maximum amount when knocking occurs, the amount of correction is limited to the preset maximum amount. In this way it is assured that the mixture combustion state will be a minimum allowable boundary state.
However, in the aforementioned prior art, the amount of correction applied to the control variable suddenly reaches the set maximum correction amount when the engine is cold and, hence, there is a decline in the combustibility of the mixture. The problem that results is an abrupt decline in engine output
The inventors have carried out extensive research into the causes of the aforementioned drop in engine output. As a result of such research, it has been clarified that since piston-cylinder clearance is comparatively large when the engine is cold, the pistons develop excessive play in their cylinders and the peripheral surface of each piston strikes the wall surface of its cylinder strongly when combustion occurs. It has been found that the noise produced by this phenomenon is erroneously detected as engine knocking and, as a result, when the engine is cold, the aforementioned amount of correction suddenly reaches the maximum value based on this erroneously detected knocking and invites a decline in engine output, as mentioned above.
Accordingly, in an effort to suppress this drop in output when the engine is cold, it has been comtemplated to forcibly halt knocking control at such time. However, knocking can occur even with a cold engine. Consequently, if knocking control were to be suspended when the engine is cold even though knocking is actually occurring, piston damage and burnout would result particularly when the engine is running under a high load, as when intake air is supercharged by an exhaust turbosupercharger or the like. Thus, the expedient of forcibly halting knocking control when the engine is cold is disadvantageous in that engine durability would suffer.